Open University Uranium-Series Laboratory  
Earth and Environmental Sciences, The Open University, Milton Keynes, UK  

Global Change  
River water  

Weathering of Ca-Mg silicates can bind a significant amount of CO2, and the importance of basalt weathering, particularly on the seafloor and volcanic islands, as a significant sink of atmospheric CO2 has been demonstrated recently (Louvat and Allègre, 1998; Brady and Gislason, 1997). U-series isotopes measured for the weathering products carried by rivers can be linked to both the age and the degree of chemical erosion at the scale of a watershed (Vigier et al., 2001). Lithium isotope ratios measured in rivers reflect the intensity of silicate weathering (Huh et al., 2001).

 

Iceland provides a unique opportunity for the study of basalt erosion in a cold oceanic climate ( 5°C to 10°C and 400 to 4000 mm.yr-1 of precipitation), and for comparing glacier fed-rivers with spring-fed rivers. More than 90% of Iceland consists of basalts, and glaciers take up about 12% of the surface area. The monolithological character allows the signature of lithology to be distinguished from fractionation due to chemical weathering, particularly for the Li isotope fractionation observed in rivers. River water samples show significant variations of (234U/238U) (from 1.65 to 1.07) and δ6Li for dissolved phases range between -16.8 and -23.3, suggesting that 6Li/7Li ratios fractionate during weathering of the source basalts (δ6Li-MORB~-4.5). High (234U/238U) and low δ6Li have been found for rivers characterised by low U and Li contents, draining old basalts (>0.7my) in areas with low weathering rates. Low (234U/238U) with high δ6Li and high U content, have been found in the area highest weathering rates of Icelandic basins.
The correlation of U-series with δ6Li strongly supports the hypothesis that both U and Li isotopes reflect the degree of silicate weathering, (Bourdon, et al., 2002).

 
Stalagmites
 
Temperature fluctuations inferred from δ18O variations in deep-sea sediment cores and in cores drilled in polar, Greenland and glacier ice, document climate change on sub-Milankovitch time scales of between 10y and 1,000y. The ice cores provide considerable temporal resolution (Grootes, et al., 1993; Mayewski, et al., 1993), but confidence in ages obtained by counting back annual layers may only be 600 y at the 12,500 y level (Alley et al., 1993). The radiocarbon timescale has been recalibrated against U-Th ages on corals back to 30 ky BP (Bard et al., 1993, Burr et al., 1998) but there are still considerable problems obtaining accurate 14C ages, particularly in calcareous lake sequences because of the 'hard-water' effect (Israelson et al., 1997).
 
The great advantage of speleothems (carbonate cave deposits) is that they preserve high-resolution records of environmental change, they are protected from erosion, and they yield high precision U-Th dates (e.g. Edwards et al., 1987). Paleoclimate indicators other then the traditional δ18O-based temperature estimates in stalagmites are being used increasingly. Luminescence variations caused by variations in organic acid concentrations and layer thickness can indicate rainfall, and trapped pollen are indicative of the local flora at the time of calcite deposition.
A major target has been an absolute chronology for the much used biostratigraphic zones in the Late Quaternary. Pollen have been separated from cave deposits that have then been dated by U-Th isotopes as part of a wider project to reconstruct palaeo-environmental changes in the Late Quaternary. New results have established that speleothems were deposited in both interglacial and interstadial periods. For example, a 4 cm flowstone sample from the Mendip Hills is bracketed by ages of 71,000±500 y to 66,300±700 y. Thus, this sample grew rapidly during the early part of the last glaciation, in oxygen isotope stage 4. However, the sample contains pollen of thermophilous species including oak, ash and hazel, suggesting that climatic conditions in the south of England at this time were not as severe as previously supposed for that period. The organic acids in this flowstone also suggest fairly wet soil conditions with little humification of organic matter (McGarry and Baker, in press)
 
Similarly, material from Stump Cross Caverns in Yorkshire has a basal age of at 118,000 ±1200 y, early in the last interglacial, and pollen that indicate a typical interglacial assemblage of pine, birch, oak, alder, ash, hazel, ivy, grass and herbs. Soil conditions were wet when flowstone growth began and humification levels increased during its growth. When more dates are combined with the pollen and organic acid data, the dated terrestrial record of vegetation change can be extended much further back in time than has previously been possible with radiocarbon dating.
 
Lake marls
 
U-series disequilibrium dating of lake sediments is often complicated due to the presence of detrital 230Th within the sample matrix. The analysis of bulk sample thus provides a 230Th signature that is comprised of detrital and authigenic 230Th, and yields an apparent age much
 
 
  Core depth vs Age for lake marls from Hawes Water, U-series dates with 2 sigma uncertainty bars, filled squares indicate the approximate ages of key indicator horizons in the pollen record    
 
older than reality. Many attempts at leaching such samples have yielded non-reproducible results. However, exciting results from lacustrine sediments from Hawes Water (NW Lancashire, UK) indicate that this particular lake has marls that are less affected by detrital thorium input than many other lakes. Hawes Water, a small shallow lake, is much more responsive to climatic and hydrological change than large deep-water lakes. U-series ages match fairly well with a proxy chronology based on pollen horizons and stable isotopic data, See Figure.
Trace element and stable isotope data for the Holocene sequence at Hawes Water indicate considerable climatic change during the last 10 ky contrary to popular belief that the Holocene in NW Europe since the Younger Dryas as been a climatologically quiet period. This is the first attempt to combine high-resolution stable isotopic and geochemical data with an absolute chronology of a NW European shallow lake.
 

Alley, R.B. et al., 1993, Nature, 362, 527-529.
Bard, E. et al., 1993, Radiocarbon, 35, 191-199.
Brady, P.V. and Gislason, S.R., 1997, Geochim. Cosmochim. Acta, 61, 965-973.
Bourdon, B. et al., 1996, Nature, 384, 231-235.
Burr, G.S. et al., 1998, Radiocarbon, 40, 1093-1105.
Edwards, R.L. et al., 1987, Science, 236, 1547-1553.
Grootes, P.M. et al., 1993, Nature, 366, 552-554.
Huh, Y. et al., 2001, Earth and Planetary Science Letters, 194, 189-199.
Israelson, C. et al., 1997, Earth and Planetary Science Letters 153, 251-263.
Louvat, P. and Allègre, C.J., 1998, Chem. Geol. 148, 177-200.
Mayewski, P.A. et al., 1993, Science, 261, 195-197.
McGarry, S. and Baker, A., (in press).
Vigier, N. et al., 2001, Earth and Planetary Science etters, 193, 549-563.
 
  To Top

© Peter van Calsteren
Last updated: December 23, 2011 11:39